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Prediction of Giant Thermoelectric Power Factor in Type-VIII Clathrate Si 46

View Article: PubMed Central - PubMed

ABSTRACT

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

No MeSH data available.


The bandstructure and position of the Fermi level (dashed line) in Na8Si46-VIII.
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f3: The bandstructure and position of the Fermi level (dashed line) in Na8Si46-VIII.

Mentions: For n-type doping, one can use intercalation with any of the alkali or alkali-earth elements. Such intercalations can dope Si46-VIII to highly degenerate levels. According to Figure 2-b, at 1021 cm−3 electron concentration, the n-type Fermi level is ~−0.06 eV below the conduction band edge. The intercalation can provide large doping concentration much larger than achievable doping levels in diamond silicon which is limited to solid solubility limit of the dopants. For example, our calculations show that the Fermi level of Na8Si46-VIII is about 0.8 eV above the conduction band edge as shown in Figure 3. This is obviously much higher than the needed Fermi energy. Therefore, partial intercalation with Na can provide the required Fermi level. In addition, as it is evident from comparison of Figure 3 and Figure 1-(c), the guest atom does not affect the band structure considerably due to its weak interaction with the cage atoms. It is known that intercalation can also reduce the thermal conductivity, which is desired for TE application. Therefore, Si46-VIII is a good parent material for designing efficient thermoelectric materials. It is also noted that intercalation has shifted the Fermi level from the valance band edge to deep inside the conduction band. Therefore, it is possible to adjust the Fermi level anywhere in the energy gap by partial intercalation of the pristine material.


Prediction of Giant Thermoelectric Power Factor in Type-VIII Clathrate Si 46
The bandstructure and position of the Fermi level (dashed line) in Na8Si46-VIII.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382702&req=5

f3: The bandstructure and position of the Fermi level (dashed line) in Na8Si46-VIII.
Mentions: For n-type doping, one can use intercalation with any of the alkali or alkali-earth elements. Such intercalations can dope Si46-VIII to highly degenerate levels. According to Figure 2-b, at 1021 cm−3 electron concentration, the n-type Fermi level is ~−0.06 eV below the conduction band edge. The intercalation can provide large doping concentration much larger than achievable doping levels in diamond silicon which is limited to solid solubility limit of the dopants. For example, our calculations show that the Fermi level of Na8Si46-VIII is about 0.8 eV above the conduction band edge as shown in Figure 3. This is obviously much higher than the needed Fermi energy. Therefore, partial intercalation with Na can provide the required Fermi level. In addition, as it is evident from comparison of Figure 3 and Figure 1-(c), the guest atom does not affect the band structure considerably due to its weak interaction with the cage atoms. It is known that intercalation can also reduce the thermal conductivity, which is desired for TE application. Therefore, Si46-VIII is a good parent material for designing efficient thermoelectric materials. It is also noted that intercalation has shifted the Fermi level from the valance band edge to deep inside the conduction band. Therefore, it is possible to adjust the Fermi level anywhere in the energy gap by partial intercalation of the pristine material.

View Article: PubMed Central - PubMed

ABSTRACT

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

No MeSH data available.